1. Field of the Invention
The present invention relates generally to the field of signal processing. More specifically, the present invention is related to pulse rate monitors capable of providing accurate measurement and display of a user's pulse rate during times of physical exercise or other activity.
2. Background of the Related Art
The accurate measurement of an active person's pulse rate at the wrist is complicated due to the artifacts produced by body motion such as when the person is running or otherwise engaging in physical activity or exercise. Therefore, pulse rate monitors presently in the market utilize chest bands that are worn close to the heart to minimize the effect of motion produced by exercise. Artifacts produced by body motion are detected by pulse rate sensors as “noise” that masks the heart rate pulse signal being measured. In order to mitigate the effects of these artifacts, it is necessary to filter out and cancel as much of the noise signal occurring in the pulse rate frequency band as possible while retaining the desired pulse signal.
Signals of interest are generated by transmitting a light source in the near infrared region into the tissue and measuring the returned signal intensity. Typically two or four light emitting diodes (LEDs) are employed with varying intensity to establish the optimum optical window. The return signal strength will be modulated by the capillary blood flow in the tissue and will vary with the physiologic pulse of the subject. This is a well understood and established principal that has been applied to pulse monitoring equipment for years. Pulse rate sensing taken at locations other than close to the heart, has not been successful because of the relatively low signal strength and relatively high “noise” content. The low signal strength can be attributed to a number of factors including variations in skin and hair density, variations in vascularization, and optical alignment. Further, the received signal includes several components which can be generally classified as “noise” when attempting to sense pulse rate. These high “noise” levels are in addition to classical noise sources present within almost all electrical systems and primarily include inherent optical noise sources, interfering light sources, and motion artifact.
To illustrate, FIG. 1A depicts a signal of interest. FIG. 1B depicts noise caused by motion artifact, interfering lights sources, random noise and the like. FIG. 1C illustrates how the signal of interest is masked by noise due to low signal strength, as an example.
While conventional signal processing techniques may be able to reduce “out of band” noise; that is, noise not found within the frequency of interest, they are challenged to address noise that mimics the signal of interest and that is non-random, the most common of which is motion.
Various pulse rate detection systems are known in the art. U.S. Pat. No. 4,338,950 to Barlow, Jr. et al. discloses an instrument comprised of wrist-mounted unit, which contains a piezoelectric transducer for detecting pulse rate and an accelerometer for detecting body motion during physical activity. The instrument further includes a processor for subtracting the body movement component from the signal, thus yielding the true heart beat signal.
U.S. Pat. No. 5,431,170 to Mathews discloses a device which may be worn on the wrist or hand during physical activity. The device contains a light sensor to measure pulse and light sensor or accelerometer for measuring movement. Mathews discloses that a noise cancellation circuit takes the values from these sensors to give a true pulse signal that is free of pedometry vibration or noise.
U.S. Pat. No. 5,807,267 to Bryars et al. discloses an apparatus which can be combined in a single unit with a wrist watch to display the user's heart pulse rate during physical exercise. A primary piezo sensor detects the heart rate pulse of a user and a background piezo sensor detects the noise from local body motion. Signals from this background sensor are digitally subtracted from the primary pulse sensor thus allegedly reducing the effects of random body noise.
U.S. Pat. No. 6,099,478 to Aoshima et al. disclosse a pulse wave detecting means comprising an LED, photo transistor, or piezoelectric microphone. Body motion detecting means detect body motion using an acceleration sensor. Aoshima et al. disclose that pulse wave extracting means subtract the output of the two sensors to give an accurate pulse rate. Assignee related U.S. Pat. No. 5,776,070 provides similar disclosures.
U.S. Pat. No. 6,129,676 to Odagiri et al. discloses a pulse rate monitor that can be assembled into a wrist watchband and used during activities, such as running. The wrist watchband contains an acceleration sensor to detect action noise and a piezoelectric microphone to detect pulse. When constant motion such as running is detected, the action noise spectrum is subtracted from the pulse wave spectrum, which is depicted in FIGS. 6A-C. Running speed and distance may also be obtained. U.S. Pat. Nos. 5,697,374 and 6,023,662 provide similar disclosures.
U.S. Pat. No. 6,361,501 B1 to Amano et al. discloses a pulse wave diagnosing device formed of device main body having a wristwatch structure, and pulse wave detection sensor unit. A body motion component remover subtracts corrected body motion data from corrected pulse wave data. Body motion waves are detected by an acceleration sensor. This device may be incorporated into a pedometer.
U.S. Patent Appln. Publn. 2005/0116820 A1 to Goldreich discloses a wrist mounted device that detects pulse rates. The vibration sensor is a piezo ceramic sensor that measures movement of the wrist and may include an accelerometer. A physiologic sensor detects the blood pressure pulse rate, and may be fiber optic. Ambient sensors may also be present.
Whatever the precise merits, features, and advantages of the above cited references, none of them achieves or fulfills the purposes of the present invention. For these reasons it would be desirable to provide an improved device and method for accurately measuring pulse rate during physical exercise and other activities.